Powdered metal article and method of making



Feb. 4, 1964 H. 'r. HARRISON 3,120,436

POWDERED METAL ARTICLE AND METHOD OF MAKING Filed March 25, 1961 NIX WHITE CAST IRON POWDER AND PURE IRON POWDER PRESS AT 15-50 TONS/SQJN.

PLACE IN PROXIMITY T0 COPPER ALLOY PASS THROUGH FURNACE PREHEAT AT 500-12 00? FlRs'r ZONE AT I650E (8 MINUTES) szcorvo 20m:

AT zooon (8 MINUTES) INVENTOR. Harold T. Harrison United States Patent Ofiice 3,120,43fi Patented Feb. 4, 1%64 3,120,436 PGWDERED METAL ARTICLE AND METHOD OF MAKING Harold T. Harrison, Worcester, Mass, assignor to The Presmet Corporation, Worcester, Mass, a corporation of Massachusetts Filed Mar. 23, 1961, Ser. No. 97,826 2 Claims. (Cl. 75-200) This invention relates to a powdered metal article and a method of making the same and more particularly to a sintered iron-carbon article which has been infiltrated with copper, or a copper base alloy.

In powder metallurgy it is well known that articles made with powdered pure iron, after compacting and sintering, have a strength in the order of 15,000 p.s.i. When such a pure iron article is infiltrated with copper the strength goes to the neighborhood of 60,000 p.s.i. When a powdered metal article is made with about 1% of carbon (Where the carbon is in combined form), the article has a strength of around 26,000 p.s.i. and, when such an iron-carbon article is infiltrated with copper, the strength goes to the neighborhood of 90,000 p.s.i. The last-named alloy, therefore, is a very desirable product. Alloys produced by powder metallurgy infiltration techniques are described in ASTM specification B-30358T and they account for a large tonnage of structural parts produced by the industry. Class B and C alloys containing definite percentages of carbon exhibit mechanical properties that are as good as those produced by any of the other powder metallurgy techniques, even as good as those techniques using double pressing and double sintering.

In many cases the economics of the process for producing iron-carbon parts that have been copper-alloy infiltrated precludes its use. One factor that contributes considerably to the expense of the processing is the need for a double furnace operation. The common method of manufacture is to mix the appropriate amount of graphite powder with pure iron powder. This mixture is compacted and then sintered to diffuse the graphite and iron. The resulting iron and iron-carbide skeleton is then infiltrated with the copper-base alloy in a second furnace operation. Infiltration is impossible until all or substantially all of the free graphite has been transformed to the iron-carbide phase by the initial sintering operation; the poor wettability of graphite by molten copper or its alloys is responsible for this condition. These and other difiiculties experienced with the prior art articles and methods have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the invention to provide a powdered metal article and a method of making it, wherein the article has a very high strength and is economical to manufacture.

Another object of this invention is the provision of an iron-carbon powdered metal article which is infiltrated with copper and which makes use of inexpensive materials.

A further object of the present invention is the provision of an iron-carbon powdered metal article, copper infiltrated, which is produced by a single-pass furnace treatment, thus omitting a handling operation, making the first piece emerging from the furnace immediately available for inspection, and saving fuel in the operation.

It is another object of the instant invention to provide a method of producing an iron-carbide powdered metal article with copper infiltration by a process which is less likely to decarburize in a furnace having a marginal atmosphere.

Although the novel features which are believed to be characteristic of this invention will be particularly pointed out in the claims appended hereto, the invention itself, as to its objects and advantages, the mode of its operation and the manner of its organization may be better understood by referring to the following description taken in connect-ion with the accompanying drawing forming a part thereof, in which:

The single figure is a flow chart illustrating the method of producing a powdered metal article using the principles of the present invention.

In a general way, the present invention accomplishes the sintering and infiltration operation in one furnace pass. This is accomplished by the use of a cast iron powder containing 4% carbon in the form of iron carbide, rather than using a graphite powder for the carbon addition as was the practice in the past. This cast iron (4% carbon) powder is diluted with commercially pure iron powder in the proper amount to obtain a mix that will result in the total carbon content desired in the finished product. Articles pressed from this mix can be infiltrated and sintered in one furnace operation inasmuch as there is no free graphite to reduce wettability of the molten copper phase. Diffusion of carbon is quite rapid and the end product is entirely homogeneous with respect to the iron carbide phase.

An example of the use of the method of the invention involved mixing a powder of white cast iron powder (having 4% carbon in combined form and no free carbon) with a powder of commercially pure iron to give an overall carbon content of 1% which resulted in around 0.8% carbon in the finished article. This powder was formed and pressed at 30 tons per square inch to form a compact article. Then, the article was passed through a furnace. The furnace is provided with a pre-heating zone in which the temperature gradually increased to 1200 F. This had the effect of expelling the zinc stearate which was used as a die lubricant in the initial mix. The article was then passed into a first zone of the furnace in which the temperature was at 1650 F., which temperature was below the melting point of the copper alloy. It should be pointed out that before passing the article through the furnace it was placed in intimate contact with a copper alloy piece of sufiicient volume to substantially infiltrate the powdered metal article, so that the molten copper or alloy would substantially fill the interstices of the iron skeleton. In any case, this first zone pre-sintered the iron part of the compact to provide a strong skeleton. Ilhis served to resist the forces of disintegration which normally takes place when liquid copper soaks into an unsintered skeleton. This effect of attempting to infiltrate an unsintered iron skeleton with copper is similar to the effect that one has when a fairly substantial cake of mud is dropped into a pail of water; the water causes the cake to disintegrate. The article is next passed into and through a second zone of the furnace at 2,000 F., which temperature is above the melting point of the copper alloy. The molten copper is drawn into the article and infiltrates it by capillary action. The finished article was then removed from the furnace. In the example, the article remained in the first zone for eight minutes and in the second zone for eight minutes. The ultimate tensile strength was 83,400 p.s.i.

'It should be pointed out that experience has shown that the white cast iron powder may contain from 3 /2% to 4%% carbon in combined form. Furthermore, the pressing operation may take place from 15 to 50 tons per square inch, although it is usually in the range of from 25 to 30 tons per square inch. The first zone of the furnace in which the pre-sintering operation takes place may have a temperature from l600 to 1850 F.; of course, the temperature of this Zone depends on the melting point of the particular copper alloy. If pure copper is used, it would be higher than an alloy of, say, zinc or tin. The

e,120,ase

temperature in the second zone may lie in the range of from 1750 to 2050 F.; in any case, it is above the melting point of the copper or its alloy.

The advantages of the process of the invention are many. First of all, the white cast iron powder is less expensive than commercially pure iron powder, so that the initial material used in producing the article are less expensive. The use of a single pass furnace treatment has several advantages. First of all, the handling operation used in a two-pass furnace operation adds considerably to the cost in the form of labor expense; with a two-pass operation it is necessary to cool the articles and then to carry them back to be passed through the furnace for the second time. Of course, the furnace temperatures and everything have to be changed between the two passes. Another advantage is that, if the operation is not going properly, this fact becomes evident as soon as the first piece comes through the furnace. With the old methods it was necessary to completely process all of the articles in a first furnace pass and then to start them passing through the second furnace pass; only then was the article completely finished and available for inspection. With the present method changes can be made in the operation on the batch before the batch has completely passed through the furnace. Another advantage is that it has been noted that an article made according to the present method is less likely to de-carburize in a furnace atmosphere which is marginal with respect to carbon potential. Since it is expensive to regulate the furnace very carefully to avoid a marginal atmosphere, the fact that the present article is less susceptible to decarburization in such an atmosphere is a decided advantage in operating commercially.

While certain novel features of the invention have been shown and described and pointed out in the annexed claims, it will be understood that various omissions, substitutions and changes in the forms and details of the invention illustrated and in the operation of the method may be made by those skilled in the art without departing from the spirit of the invention.

The invention having been thus described, what is claimed as new and desired to secure by Letters Patent is:

l. A method of forming a powdered metal article, comprising the step of mixing white cast iron powder with pure iron powder, pressing the resulting powder mixture to obtain a formed article, placing the formed article in contact with a quantity of copper in the solid state, and passing the article and copper through a furnace having a first zone at a temperature below the melting point of the copper and a second zone at a temperature above the melting point of the copper.

2. A method of forming a powdered metal article, comprising the steps of mixing an iron powder having 4% by weight of carbon in combined form with pure iron powder in an amount sufiicient to give between 0.2% to 1.25% by weight of carbon in the article, pressing the mixture of powders at a pressure in the range of from 15 to 50 tons per square inch to obtain a formed article, placing the formed article in physical contact with a quantity of copper in the solid state, and passing the formed article and the copper in contact therewith through a furnace having a first Zone at a temperature in the range from 1600 to 1850 F., to produce at least partial sintering of the formed article, and a second zone at a temperature in the range from 1750 to 2050 F., to cause infiltration of the copper into the formed article.

References Cited in the file of this patent UNITED STATES PATENTS 2,198,254 Loering Apr. 23, 1940 2,422,439 Schwarzkopf June 17, 1947 2,706,694 Haller Apr. 19, 1955 2,759,846 Vosler Aug. 21, 1956 2,778,742 Shipe Jan. 22, 1957 2,851,354 Scanlan et al. Sept. 9, 1958 FOREIGN PATENTS 308,819 Great Britain Feb. 20, 1930 720,050 Great Britain Dec. 15, 1954 

1. A METHOD OF FORMING A POWDERED METAL ARTICLE, COMPRISING THE STEPS, OF MIXING WHITE CAST IRON POWDER WITH PURE IRON POWDER, PRESSING THE RESULTING POWDER MIXTURE TO OBTAIN A FORMED ARTICLE, PLACING THE FORMED ARTICLE IN CONTACT WITH A QUANTITY OF COPPER IN THE SOLID STATE, AND PASSING THE ARTICLE AND COPPER THROUGH A FURNACE HAVING A FIRST ZONE AT A TEMPERATURE BELOW THE MELTING POINT OF THE COPPER AND AT A SECOND ZONE AT A TEMPERATURE ABOVE THE MELTING POINT OF THE COPPER. 